Changes in the Hydrogen-Bonded Structure of Lawsonite: An Experimental Study to 2.5 GPa and 400 ℃

In-situ Raman spectroscopic measurements of lawsonite in the OH and H2O stretch-ing region were conducted up to approximately 2.5 GPa and 400 ℃ with a hydrothermal diamond-anvil cell. In the experimental design at room temperature, the ratio of band-area and the width at half maximum of the decomposed Raman band shows discontinuities at 0.8 GPa, and correspondingly, the 695-cm-1 peak in the low-wavenumber region was split into two peaks. In the experiments at high temperature, the splitting of the 695-cm-1 peak was induced at 2.4 GPa and 250 ℃. These findings demonstrate the existence of discontinuities in the pressure response of the strength of the hydrogen bonds, which suggests a possible change of the hydrogen-bonded structure in lawsonite under these conditions.